1. Field of the Invention
The present invention relates to a printer for recording color images on a recording medium.
2. Description of Related Art
Various techniques are known in relation to printers each of which can record a color image on a recording medium. For example, JP-A-8-174876 discloses a one-path type color thermal printer having three thermal heads that can record colors of yellow, magenta, and cyan, respectively. The three thermal heads cooperate with each other to record a color image on a recording paper unwound from a recording paper roll and being conveyed in a paper feed direction.
JP-A-9-99572 discloses a three-path type color thermal printer. In this printer, a portion of a heat-sensitive color recording paper of a length corresponding to one frame, unwound from a recording paper roll, is conveyed forward so as to be opposite to one thermal head, and then conveyed backward. In the backward conveyance, a yellow image is recorded on the portion of the recording paper. Afterward, such forward and backward conveyances of the recording paper are repeated alternately. A magenta image is recorded in the second backward conveyance. A cyan image is recorded in the third backward conveyance.
JP-A-2001-246769 discloses a one-path type color thermal printer having three thermal heads for yellow, magenta, and cyan. A recording paper unwound from a recording paper roll is conveyed in a paper feed direction. After completion of preparation for printing, the recording paper is conveyed backward. In the backward conveyance, one frame of color image is recorded on the recording paper. In this printer, after the one frame of color image is recorded, the recording paper is again conveyed in the paper feed direction and then the portion of the recording paper on which the one frame of color image has been recorded is cut off at the rear end of the color image. After the portion of the recording paper on which the one frame of color image has been recorded is discharged, the remaining portion of the recording paper, which has completed preparation for printing is again conveyed backward to record thereon the next one frame of color image.
In the printer disclosed in JP-A-8-174876, a color image can be recorded, even without conveying the recording paper in both the forward and backward directions, with conveying the recording paper in only one direction. In this printer, however, if the three thermal heads are pressurized onto the recording paper in the order of the thermal heads the printing position of which the recording paper has reached, the image to be recorded may be deteriorated due to variation of load in the conveyance of the recording paper. For this reason, such a printer is designed so as to start recording a color image after all the three thermal heads are pressurized onto the recording paper. As a result, any image can not be recorded in a region near the leading edge of the recording paper between the most upstream thermal head and the most downstream thermal head in the conveyance direction.
Contrastingly, the printer disclosed in JP-A-9-99572 or JP-A-2001-246769 can record a color image even in a region near the leading edge of the recording paper. Thus, the recording paper is hardly wastefully consumed. In addition, in the printer disclosed in JP-A-2001-246769, because all the three thermal heads are being pressurized onto the recording paper while the recording paper is conveyed backward, the image to be recorded is hardly deteriorated due to variation of load in the conveyance of the recording paper.
In the printer disclosed in JP-A-9-99572, however, the conveyance of the recording paper in either of the forward and backward directions must be repeated three times for recording one frame of color image. Similarly in the printer disclosed in JP-A-2001-246769, the conveyance of the recording paper in either of the forward and backward directions must be carried out every time when one frame of color image is recorded. Therefore, when a plurality of frames of color images are recorded with such a printer, the conveyance of the recording paper in either of the forward and backward directions must be repeated many times. More specifically, in the case of recording m frames of color images (m: an integer of two or more), the recording paper must be conveyed in either of the forward and backward directions 3m times in the printer disclosed in JP-A-9-99572, and m times in the printer disclosed in JP-A-2001-246769.
In the printer disclosed in JP-A-9-99572 or JP-A-2001-246769, a pair of conveyance rollers are disposed between the recording paper roll and the thermal heads. The conveyance direction of the recording paper is changed to one of the paper feed direction and the backward direction by switching over the driving direction of the pair of conveyance rollers. Therefore, in the case of the printer disclosed in JP-A-9-99572, the driving direction of the pair of conveyance rollers must be switched over (6m−1) times. In the case of the printer disclosed in JP-A-2001-246769, the driving direction of the pair of conveyance rollers must be switched over (2m−1) times.
Such an operation of switching over the driving direction of the pair of conveyance rollers brings about a time loss. Therefore, if the number of times of switching over the driving direction of the pair of conveyance rollers increases, the total time loss increases accordingly. As a result, the processing performance of the printer lowers. Thus, the techniques disclosed in JP-A-9-99572 and JP-A-2001-246769 can not realize a high processing performance.
As image recording methods each using a thermal head, there are a thermal transfer method and a thermal recording method. In the thermal transfer method, which is further classified into a phase change type and a dye sublimation type, an ink ribbon is interposed between a thermal head and a recording paper and ink applied on the ink ribbon is selectively transferred onto the recording paper by the thermal head to record an image on the recording paper. Contrastingly, in the thermal recording method, a recording paper having heat sensitivity is used. Thus, an image can be recorded on the recording paper by using no ink ribbon. In either method, during a printing operation, the thermal head must be brought into close contact with the ink ribbon in the thermal transfer method or the recording paper in the thermal recording method without interposing any air layer, or must be very close to the ink ribbon or the recording paper. Such a thermal head includes a large number of heating elements that are arranged in a row and can be selectively electrified. It is known that such a thermal head may break down due to overheating if heating elements are electrified while the thermal head is not opposite to a recording paper or if the heating elements are not being opposed to the recording paper immediately after the electrification is stopped.